1. Field of the Invention
The present invention relates to Radio-frequency identification (RFID).
2. Description of the Related Art
RFID is the use of an object, which is typically referred to as an RFID tag, for the purpose of identification and tracking using radio waves. Generally, the RFID tag is physically attached to a product, person, or other object. RFID tags can be categorized into three basic types: active, passive, and semi-passive.
An active RFID tag is a wireless device that is powered locally and contains a local radio frequency (RF) receiver and a source of RF power that actively transmits to a remotely located network node. An active RFID tag can be powered from a local battery or from an external source, such as the AC power grid or a photovoltaic solar panel. The present state of the art for a wireless network consisting of active RFID tags permits the network to be extended over many miles. For example, the current state of the art for an active ZigBee circuit transponder operating at 915 MHz operating within the US FCC Part 15 RF emission limits with an appropriate antenna is approximately 10 miles. ZigBee is a low-cost, low-power, wireless mesh networking standard.
A passive RFID tag is a wireless device that is powered by RF power received from a nearby interrogator. A passive RFID tag contains a local radio receiver but it communicates with the interrogator only by modulating and reflecting the RF radiation provided by the interrogator. This method is sometimes referred to as modulated backscatter.
The passive RFID tag receives wireless data from the interrogator by demodulating an RF carrier received from the interrogator and decoding the data from the demodulated signal. The passive RFID tag transmits data to the interrogator by reflecting the RF radiation provided by the interrogator and modulating data onto the reflected radiation by adjusting the load impedance of the tag antenna. The interrogator decodes the data from the reflected radiation.
In a fully passive RFID tag there are no local power sources such as batteries, solar cells, solar panels, piezoelectric generators, or the power grid. A fully passive RFID tag can be powered by a circuit that scavenges power from incident RF originating from an interrogator. The present state of the art for a passive RFID network permits a separation between the passive RFID tag and a wireless interrogator of up to 60 meters.
A semi-passive RFID tag provides the passive tag function as a subset of its total function. However, a semi-passive RFID tag also receives power locally. During normal operation the semi-passive RFID tag does scavenge power from the remote interrogator. The semi-passive RFID tag is usually powered locally from a battery, the power grid, solar cells, solar panels, wind generation source, vibration scavenging source, piezoelectric power source, or some other power source. The local power supply is given priority in supplying power to the tag by a local prioritizing power control circuit. When the local power supply is not adequate the prioritizing power-control circuit gates in power to the passive tag by scavenging from the RF incident energy supplied by the remote interrogator device.
Traditional RFID systems use tags containing digital memory. The digital memory within the tag can be read from or written to by an interrogator. These traditional systems do not include sensors to determine the state or condition of other devices, and are thus limited to applications that require only storage and retrieval of data, such as inventory control systems. Additionally, traditional RFID tags include only one type of RFID circuit, and thus, must either include a power source to constantly supply power to an active RFID circuit, or are limited to applications requiring only a passive RFID circuit.
Therefore, there is need in the art for a more complex RFID system. Specifically, there is a need for a system including an RFID tag that contains both an active and a passive circuit. Additionally, there is a need for an RFID tag that can sense, control, log, and monitor electrical and identification parameters from a power source or a power load device. The present invention satisfies these and other needs.